SOI (SOUTHERN OSCILLATION INDEX)

The 6-month average of the Southern Oscillation Index was -0.9 during the second half of 2006. This is consistent with weak El Niño conditions present at that time. Since then, the SOI has risen slightly, and its values stood at -1.1, -0.5, and -0.4 during January, February and March 2007, respectively. While the eastern equatorial Pacific Ocean temperature anomalies have gone from warm to cold during the first quarter of 2007, the SOI has not reversed sign, and remains in negative territory. If the ocean continues to cool, and La Niña conditions develop (as some computer models are now indicating), then the SOI should rise and become predominantly positive in the coming months. If ENSO-neutral conditions prevail, then the SOI should fluctuate near zero for the foreseeable future.

*Note: The SOI is defined as the normalized pressure difference between Tahiti and Darwin, Australia. There are several slight variations in the SOI values calculated at or other sites representative of the western and eastern tropical Pacific, respectively. The historical record of the SOI from 1866 to present was calculated based on the method given by Ropelewski and Jones (1987). The complete table can be found at http://www.cru.uea.ac.uk/cru/data/soi.htm.

Ropelewski, C.F. and Jones, P.D., 1987: An extension of the Tahiti-Darwin Southern Oscillation Index. Monthly Weather Review 115, 2161-2165.